In this kind of technical field, research and development on the next generation mobile communication system including radio access schemes, retransmission control, handover and the like are being conducted at high speed.
FIG. 1 shows an example of a retransmission control procedure. The right side of the figure shows operations of an entity (TxMAC) of the medium access control (MAC) sublayer and an entity (TxRLC) of the radio link control sublayer in the transmission side. The left side of the figure shows operations of an entity (RxMAC) of the medium access control (MAC) sublayer and an entity (RxRLC) of the radio link control sublayer in the reception side. In a downlink, a transmitter is a base station, and a receiver is a user apparatus. In an uplink, the transmitter is a user apparatus and the receiver is a base station.
In steps S21 and S22, a packet to be transmitted is prepared. From the MAC sublayer to the RLC sublayer, a packet to be transmitted is requested (new data request). A transmission packet is prepared in the MAC sublayer according to this request. In the example shown in the figure, a sequence number “0” is provided to this packet, that is, a packet data unit (PDU) (SN=0).
As indicated in step S11, the packet prepared in the transmission side is transmitted to the reception side. In this case, the packet data unit including user data specified by the sequence number (SN) is transmitted by a data channel, control information such as user identification information (UE-ID), process number (Proc) and a new data indicator (NDI) is transmitted by a control channel, and one that is broadcasted by a broadcast channel is used as a system frame number (SFN) indicating absolute transmission timing in the cell.
The receiver that receives the control channel and the data channel together with the broadcast channel executes error detection for the received packet using a cyclic redundancy check (CRC) method, for example. The error detection result indicates being negative (NACK) or being positive (ACK). The former indicates that an error exceeding an allowable range is detected, and the latter indicates the inverse. In the example shown in the figure, an error is detected (CRC:NG).
As indicated in step S12, an error detection result is reported to the transmission side. In response to a negative error detection result, the transmission side specifies a packet related to the negative acknowledgement to retransmit the packet. The packet transmitted from the transmission side is stored in a buffer (retransmission buffer) even after radio transmission, and is discarded when a positive error detection result (ACK) is obtained. Therefore, when a negative error detection result is reported, a packet that was transmitted before is specified according to the negative error detection result, and the specified packet is retransmitted.
In the example shown in the figure, since a negative acknowledgement (NACK) is transmitted in step S12, the transmission side also should recognize it and proper retransmission should be performed. However, due to status of the radio link, there is a case in which the transmission side (TxMAC) performs erroneous recognition as if a positive error detection result were reported. In the example in the figure, although the reception side returns the negative acknowledgement, the transmission side proceeds with its process as if a positive acknowledgement were reported.
As a result, a subsequent another packet is prepared in steps S23 and S24, and the packet is transmitted from the transmitter to the receiver as indicated by step S13. In the example shown in the figure, a packet of SN=4 is radio-transmitted at a timing of SFN=8 together with control information of Proc=0 and NDI=1.
The receiver refers to the process number (Proc) and the new data indicator (NDI) and the like to ascertain that a new packet that is not a retransmission packet is transmitted even though returning a negative acknowledgement in the past. As a result, it can be detected that a negative acknowledgement is erroneously processed as a positive acknowledgement.
As indicated by step S14, according to this error detection result, an indicator including a system frame number (SFN=3) of the packet of the retransmission subject is generated, and is reported to the transmission side. The transmission subject is the packet for which the error detection result was the negative acknowledgement, and it can be identified by the error detection result performed after step S11 and SFN. This indicator may be referred to as False Ack Indicator (FAI).
The transmitter extracts a system frame number (SFN=3) included in the reported indicator. A sequence number of a packet transmitted at the timing of the system frame number (SFN=3) is 0. This is stored in the transmission side. The sequence number of the packet to be retransmitted is specified from the reported system frame number in the above-mentioned way.
As indicated in step S25, the specified sequence number (SN=0) is reported to the TxRLC sublayer that manages the transmission packet, and after that, the packet is retransmitted to the reception side. For the sake of simplifying the figure, steps after the step S26 are not shown.
Accordingly, even though erroneous recognition of the error detection result (ACK/NACK) occurs in the transmitter side, the reception side can specify a packet that should be retransmitted so that proper retransmission control can be performed. Such a method is described in the non-patent document 1, for example.    [Non-patent document 1] R2-060907, “MAC functions: ARQ”, Samsung